Transcript Lecture 7: Energy Toolkit III

Energy, Society,
and
the Environment
Unit IV: How Do Power Plants Work?
Combustion and Heat Engines
Outline
• Combustion: Energy Generation and
Pollutants
• 1st Law of Thermodynamics
• 2nd Law of Thermodynamics
• Efficiency
Combustion
• What do we do with fossil fuels: burn them
– Combustion: impacts
• Fuels
• Balancing combustion chemical equations
• Combustion products
What happens in combustion?
• Fuel + oxidizer -> Products + light + heat
• Combustion, in its simplest form, e.g: methane
CH4 + 2O2  CO2 + 2H20
A clean reaction, except for the issue of carbon dioxide and
the global climate
This idealized reaction takes place in an ‘atmosphere’
(oxygen) free of impurities
How much CO2 is produced when 1 ton of cellulose (C6H12O6) is burned?
(Cellulose is the primary component of plant matter, wood etc)
Write the equation:
C6H12O6 + O2  CO2 + H2O
Balance first the carbon:
C6H12O6 + O2  6CO2 + H2O
Then the hydrogen:
C6H12O6 + O2  6CO2 + 6H2O
Last, the oxygen (because you can change the oxygen without altering other elements):
C6H12O6 + O2  6CO2 + 6H2O,
So, 6 + x = 18  x = 12, or 6 O2
The balanced equation is:
C6H12O6 + 6O2  6CO2 + 6H2O
How much CO2 is produced when 1 metric ton of wood is burned,
continued ...
1 molecule of cellulose produces 6 molecules of CO2
How much do these weigh?
Remember your atoms
C = 6 p + 6 n = 12
(atomic mass unit, OR
1 mole of C is 12 grams)
O = 8 p + 8 n = 16
How much CO2 is produced when 1 metric ton of wood is burned,
continued ...
1 molecule of cellulose produces 6 molecules of CO2
How much do these weigh?
C6H12O6 = (6 x 12) + (12 x 1) + (6 x16) = 180 grams (in 1 mole)
6 CO2 = 6 x [(1x12) + (2 x16)] = 264 grams (from 1 mole of cellulose)
So, from C6H12O6  6CO2 + 6H2O, we see that:
180 grams cellulose  264 grams CO2
106 grams cellulose  ?? grams CO2
264g x 106 g
6
=

1.47x
10
g  1.47 tons of CO
??
2
180g
Simple combustion equation,
but put it in air
• Example: methane reacts with air
CH4 + (O2 + 3. 76N2) -> CO2 + H2O + N2
(this is termed the ‘unbalanced’ version)
CH4 + 2(O2 + 3. 76N2) -> CO2 + 2H2O + 7.52N2
(balanced: exactly the correct amount of oxidizer to convert all C to
CO2, and all H to H2O)
Note: Air is 78% nitrogen, 21% oxygen, + other stuff
Real Combustion
• If combustion occurs without complete oxidation, we get instead:
CH4 + O2 + N2  mostly (CO2 + 2H20 +N2)
+ traces (CO + HC + NO...)
• This can occur when:
– temperature too low
– insufficient O
– combustion too rapid
– poor mixing of fuel and air, etc. ...
Real, Real Combustion
• At higher temperatures, N reacts with O:
air(N2 +O2) + heat  NOx (nitrogen oxides, x can be 1 or 2)
So much for pure fuels, now add impurities:
enter N, S, metals and ash (non-combustibles)
What we really get:
• Fuel (C, H, N, S, ash) + air (N2 +O2) 
(CO2, H2O, CO, NOx, SOx, VOCS, particulates) + ash
– Volatile Organic Compounds: VOCs
Real, Real Combustion and Emissions
NOx + VOCs = SMOG
CO2, NOx, SOx + oxidants + water = ACID RAIN
Particulates (especially ultrafine):
• Create inflammatory response
• Affect heart rate variability
• Inducing cellular damage
• May be associated with premature death
Emissions Controls
• Clean Air Act requires EPA to set National
Ambient Air Quality Standards (NAAQS) for
seven pollutants (referred to as criteria pollutants):
carbon monoxide (CO), lead (Pb), ozone (ground
level), nitrogen oxides (NOx), particulate matter
(PM2.5 and PM10), sulfur oxides (SO2). Last
amended in 1990.
National Ambient Air Quality Standards
Pollutant
Metric
Fed. Std.
10 mg/m
3
CA Std.
Effects
9 ppm
Angina, pre-natal
asthma
Respiratory dis.
CO
8 hr
NO2
Ann. mean 40 mg/m3
20 ppm
PM10
Ann. Mean 50 g/m3
30 mg/m3 Resp. disease,
PM 2.5
SO2
Ann. Mean 15 mg/m3
Ann. Mean 80 g/m3
in review
Resp. disease,
in review
wheez, plant dam.
Criteria Pollutants & the Clean Air Act
POLLUTANT
CO
Pb
O3 [1 hr]
PM10
PM2.5
SO2
NOx
Emissions
‘83 - ‘02
- 41%
- 93 %
- 40 %
- 34%
- 33%
- 15%
‘93 - ‘02
- 21%
-5%
- 25%
- 22%
- 17%
- 31%
-9%
Air Concentration
‘83 - ‘02
- 65%
- 94%
- 22%
-34%
- 2%
‘93 - ‘02
- 42%
- 57%
- 2%
- 10%
- 8%
- 39%
+.5%
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Announcements 2/16
• No Class THIS Friday (note the change)
• Homework #3 posted today on D2L, due
next Monday
POWER PLANTS are
Heat Engines
Using Heat to Do Work
(a bit of Thermodynamics)
What use is thermodynamics?
• For different energy sources compare:
– Efficiency
– Amount of fuel needed
– Pollution produced
• Use as a tool for improving energy systems
– Analyze each part of a power plant (pumps, turbine,
heat exchangers, etc.)
• Analyze alternative energy scenarios
– Ethanol, biodiesel, hydrogen
Some Questions That Can Be Answered
• How many fewer power plants would need
to be built in Arizona if we increased our
efficiency by 10% by 2020?
• How much could SO2, CO2, and other
pollutants be reduced by that efficiency
increase?
Coal Fired Steam Power Plant
1st Law of Thermodynamics
Water in
Water out
1st Law of Thermodynamics
• Conservation of Energy Principle
Ein
Esystem
Esystem = Ein - Eout
Eout
Earth Energy Balance
Energy from Sun
EARTH
Energy
Stored
Energy Radiated to Space
Earth Energy Balance
Energy from Sun
EARTH
Energy
Stored
Energy Radiated to Space
Estored = Ewind + Eplants + Eheat + etc. = Esun - Elost
Steady-State Condition
• Under steady-state conditions, there is no
change in the stored energy of the system.
Esystem = 0 = Ein – Eout
or Ein = Eout
Energy Balance for a Power Plant
Efuel = 1000 MJ
1000 MJ
Energy from Fuel
Power
Plant
Energy Balance for a Power Plant
Efuel = 1000 MJ
1000MJ
Energy from Fuel
Power
Plant
350MJ
Useful Energy (Work)
Efuel = Euseful + Ewaste
Euseful = 350 MJ
Energy Balance for a Power Plant
Efuel = 1000 MJ
1000MJ
Energy from Fuel
Power
Plant
350MJ
Useful Energy (Work)
Euseful = 350 MJ
Ewaste = 650 MJ
650MJ
Wasted Energy
Efuel = Euseful + Ewaste
1000 MJ = 350 MJ + 650 MJ
Power Balance for a Power Plant
1 W = 1 J/s
1000MW
Power
Plant
350MW
650MW
Pin = Pout
Pout = Pout 1 + P out 2
Pfuel = Puseful + Pwaste
Power Balance for a Power Plant
Pin = 1000 MW
Pout = 350 MW + 650MW
1000MW
Power
Plant
350MW
650MW
Pin = Pout
Pfuel = Puseful + Pwaste
Power Plant
Parts of a power plant
• Energy Source
– Combustion: Coal, natural gas, oil, biomass, garbage
• Heat Generated
– Boiler: Coal burned, heats water, creates steam
– Combustion Chamber: CH4 burned, hot gases
• Work Produced
– High pressure steam or hot gases turn turbine blades
• Wasted Heat Removed/Exhaust Treatment
– hot water dumped into a body of water or cooling tower
– exhaust gases dumped into atmosphere
• Some waste heat can be recovered
Connection to Environment
• Fuel Side: Mining, drilling, transporting
• Waste Heat Side:
– Increase temperature of body of water
• Affect fish, algae blooms, etc.
– Pollutants in waste heat stream
• Air pollutants
• Pollutants in waste water stream
• Environmental justice
– Location, impact & management of power plants
Heat Engine
• A device that converts heat into mechanical energy
• Used to approximate thermal systems
Heat Engine
High Temp. Source of Heat
Qhot
Heat
Engine
Wnet
Qcold
Low Temp. Sink of Waste Heat
Energy Balance: Qhot = Wnet + Qcold
Definitions
High Temp Source of Heat: This is the source of energy that drives the power
plant (heat of combustion, geothermal heat source, nuclear reactor, etc.).
Qhot: This is the heat transferred from the hot source.
Heat Engine: This includes the working parts of the power plant
(including pumps, turbines, heat exchangers, condensers, etc.).
Wnet: This is the net amount of work that exits the power plant. A turbine
generates energy, but the pumps and compressors use energy.
Qcold: This is the rejected or waste heat, which is dumped to a cold source (i.e.
river, atmospheric air, lake, etc.).
Low Temp Sink of Waste Heat: This is the reservoir (river, air, lake, etc.) that
the waste heat is dumped into (often goes through a cooling tower).
Efficiency
what you want
Efficiency =
what you pay for
work done
=
energy put into the system
(Several names:
I =1st Law, Actual, or Thermal Efficiency)
 =
Wnet
Qhot
=
Qhot-Qcold
Qhot
Energy Balance for a Power Plant
Power
Plant
1000MJ
Energy from Fuel
350MJ
Ein=1000 MJ from fuel
Eout=350 MJ useful energy
+650 MJ wasted energy
Useful Energy (Work)
650MJ
Wasted Energy
 = Wnet/Qhot
 = 350 MJ / 1000 MJ = 0.35 = 35%
Note: Qhot = Qin = Qfuel
Second Law of Thermodynamics
• Order tends to disorder, concentration tends to
chaos
• No process can occur that only transfers energy
from a cold body to a hot body (heat must flow
from hot to cold)
• No process can occur that converts a given
quantity of thermal energy into an equal quantity
of mechanical work (always some degradationalways some wasted energy)
2nd Law of Thermodynamics,
alternate statements:
• states in which direction a process can take
place
– heat does not flow spontaneously from a cold to
a hot body
– heat cannot be transformed completely into
mechanical work
– it is impossible to construct an operational
perpetual motion machine
2nd Law of Thermodynamics and
Carnot Efficiency
• 2nd Law: Heat cannot be converted to work without
creating some waste heat.
• What does this mean for a heat engine?
Wnet < Qhot ALWAYS
In other words, efficiency is always less than 100%.
Well, how much less?
2nd Law of Thermodynamics and
Carnot Efficiency
Carnot Efficiency: The net work produced and the heat into the system
only depend on temperatures. No thermal system can be more efficient
than the Carnot efficiency.
Qcold Tcold

Qhot
Thot
c =
Qhot - Qcold
Qhot

=1-
Qcold
Qhot
=1-
Tcold
Thot
This is the best you can achieve (under ideal conditions)
2nd Law of Thermodynamics and
Carnot Efficiency
Important: Temperatures must be in units of Kelvin.
K = ºC + 273.15
1 ºC = 1.8 F
Example: In the power plant we considered earlier, Thot = 900 K
(very hot steam) and Tcold = 300 K (room temperature). What is its
Carnot efficiency?
 c = 1 - Tcold / Thot = 1 - (300/900) = 0.67 = 67%
If this plant were an ideal heat engine, 33% of the energy would
be lost (to nearby water or to atmosphere via cooling tower)
Efficiencies of the Power Plant
Ein=1000 MJ from fuel
Eout=350 MJ useful
energy
1000MJ
Energy from Fuel
Thigh=900 K
Power
Plant
+650 MJ wasted energy
350MJ
Useful Energy (Work)
650MJ
Wasted Energy
Tlow =300 K
I= Wnet/Qhigh=350 MJ / 1000 MJ = 0.35 = 35% Reality
 c = 1 - Tcold / Thot = 1 - (300/900) = 0.67 = 67% Ideal
Comparison of Efficiencies
Type
Th (K) Tc (K)
Coal
800
300
Carnot
Eff.
62.5%
Actual
Eff.
35%
Nuclear
1200
300
75%
35%
GeoThermal
525
350
33%
16%
Waste Energy
Note: When the working fluid reaches Tlow no
more energy can be used – although the
working fluid still contains energy
(typically > 60% of Efuel)
Heat Pollution
The circulation rate of cooling water in a typical 700 MW coal-fired
power plant with a cooling tower amounts to about 71,600 cubic metres
an hour (315,000 U.S. gallons per minute) and the circulating water
requires a supply water make-up rate of about 5 percent
(i.e., 3,600 cubic metres an hour).
If that same plant had no cooling tower and used once-through
cooling water, it would require about 100,000 cubic metres an hour
and that amount of water would have to be continuously returned
to the ocean, lake or river from which it was obtained and
continuously re-supplied to the plant.
Discharging such large amounts of hot water may raise the
temperature of the receiving river or lake to an unacceptable
level for the local ecosystem.
Cogeneration
• Use waste energy in another application
• e.g., Heaters in cars use waste energy from the engine
• Easy uses: Space and water heating, especially if small
power plants can be built near urban areas: increase
total efficiency from ~ 35% to ~ 70%
• Take the Qcold through another cycle with Qcolder
• Great potential in reducing fuel use and environmental impacts
(by bringing Qcolder as close to atmospheric temperature as
possible)
Gasoline and Diesel Engines
Gasoline engines in our cars are also heat engines
called internal combustion engines
Fuel combined with gas in a closed chamber and ignited:
combustion proceeds at a very high temperature and rate
Typically 25% efficiency (mechanical energy/combustion
energy), lots of nasty pollutants
Diesel engine also an internal combustion engine
Fuel and air mixed differently than gasoline engine
Air compressed for heating, no electric spark
Combustion temperature even higher than gasoline engine
Efficiency > 30%
Less CO emission, more NOx and particulate emissions